Module 6

Question 1

Compare the two models of the Benzene ring

Answer 1

Kekulé’s structure - the six carbon’s in the ring each have one single bond, and one double bond with another carbon atom. Delocalised structure - The overlapping p orbitals causing a pi ring system of delocalised above and below the plain of the bonded carbon atoms.

Question 2

Explain why Kekulé’s structure is no longer regarded as the correct structure for Benzene

Answer 2

Bond lengths - Kekulé’s structure would imply that the single and double bonds between the carbons would be different lengths, but they are all the same length throughout the ring, by being measured using X-rays. Bond enthalpy - If Kekulé’s structure was correct, then the enthalpy change to make benzene to into cyclohexane is would be much lower than it is, suggesting that benzene is more stable than what Kekulé’s structure would suggest. Reactivity - Kekulé’s structure would suggest that it would ready react with halogens, and therefore decolourise bromine water; however, it will only react with halogens with a halogen carrier, suggesting that it doesn’t have any double bonds.

Question 3

Describe the reaction and conditions required for nitric acid to react with benzene.

Answer 3

Conditions: Concentrated Nitric acid and a Concentrated sulfuric acid catalyst. Nitration is an electrophilic substitution, with NO_2^+ being the electrophile.

Question 4

Describe the reaction and conditions required for a halogen to react with benzene.

Answer 4

Conditions: A halogen present with a halogen carrier. Halogenation is a electrophilic substitution, with X^+ being the electrophile.

Question 5

Describe a way of adding more carbon chains to a benzene ring.

Answer 5

Conditions: A haloalkane or acyl chloride in the presence of a halogen carrier. The halogen from the haloalkane/acyl chloride is polarised by the halogen carrier and breaks of, leaving a negative halogen which removes a hydrogen, and the carbon chain then attaches to the benzene ring.

Question 6

Explain the relative resistance of bromination of benzene to alkenes.

Answer 6

The electron density of the delocalised pi ring in benzene is less than that in a pi bond in an alkene, so is unable to polarise a bromine molecule, unlike alkenes.

Question 7

Explain how phenols can be shown to be slightly acidic.

Answer 7

Phenols can partially dissociate in water, releasing a proton, allowing it to react with a strong base; however, it is not strong enough to react with a weak base such as a carbonate.

Question 8

Describe the reaction and conditions required for bromine to react with a phenol

Answer 8

Conditions: No halogen carrier required. Bromination is an electrophilic substitution reaction. Bromine reacts with 2/4 directing and can easily brominate a phenol three times forming 2,4,6 - tribromophenol.

Question 9

Describe the reaction and conditions required for nitric acid to react with a phenol.

Answer 9

Conditions: Neither a catalyst nor concentrate nitric acid is required. Dilute nitric acid is sufficient. Nitration is an electrophilic substitution reaction. The nitric reacts with 2/4 directing. As NO_2^+ is an electron-withdrawing group, no further reaction after being nitrated once often takes place.

Question 10

Explain why phenols act more readily with electrophiles than benzene.

Answer 10

The p-orbital in the oxygen in the phenol donates its loan pair of electrons to the pi ring, giving it a greater electron density, so is able to polarise molecules and attract them more readily than benzene can.

Question 11

Explain the effect of electron donating groups being attached to a benzene ring and give a few examples.

Answer 11

An electron donating group is 2- and 4- directing and ‘activates’ the ring, by making it more electron dense. E.g. -OH, NH_2.

Question 12

Explain the effect of electron withdrawing groups being attached to a benzene ring and give a few examples.

Answer 12

An electron withdrawing group is 3- directing and ‘deactivates’ the ring, by making it less electron dense. E.g. -NO_2

Question 13

Explain the effect of a halogen being attached to a benzene ring.

Answer 13

A halogen is 2- and 4- directing, but does not ‘activate’ the ring like electron donating groups, as it keeps still keeps its own electron density.

Question 14

Describe the reaction and conditions for the oxidation of an aldehyde.

Answer 14

Conditions: Oxidising agent K_2Cr_2O_7 with a di/triprotic acid under reflux. RCOH + [O] —> RCOOH.

Question 15

Describe the reaction and conditions for the reduction of carbonyl compounds.

Answer 15

Conditions: Reducing agent Warmed NaBH_4(aq). Nucleophilic substation RCOH + 2[H] —> RCH_2OH

Question 16

Describe the reaction and conditions for the reaction of hydrogen cyanide with carbonyl compounds.

Answer 16

Conditions: HCN made by NaCN with H_2SO_4. Nucleophilic substitution RCOH + HCN —> RCHOHCN (a hydroxynitrile)

Question 17

Describe the test for a carbonyl group and how this can determine which carbonyl group is present.

Answer 17

Add a few drops of an unknown compound to 2,4-dinitrophenylhydrazine, and if an orange precipitate is formed, a carbonyl group is present. Filter out the precipitate and compare the melting point of the 2,4-DNP derivative to a data table to determine which carbonyl compound was present.

Question 18

Describe the test to distinguish between an aldehyde and a ketone and give the equation.

Answer 18

An equal amount of the carbonyl and Tollen’s reagent is added to a test tube and left in a 50 degree water bath for around fifteen minutes. If a ‘silver mirror’ is seen, the silver ions in the reagent have been reduced as the aldehyde as been reduced. Ag^+(aq) + e^- —> Ag(s) RCOH + [O] —> RCOOH

Question 19

Draw the mechanism of the nitration of benzene.

Answer 19

See drawing below. The use of the catalyst must be shown

Question 20

Draw the mechanism for the halogenation of benzene

Answer 20

See the below mechanism. The use of the catalyst must be shown.

Question 21

Draw the mechanism of the reduction of a ketone to form an alcohol

Answer 21

Nucleophilic addition reaction, with the nucleophile being an H^- from NaBH_4, with subsiquent protonation from water of the organic intermediate.

Question 22

Explain the water solubility of carboxylic acids

Answer 22

The -OH part of the carboxyl group can form hydrogen bonds with that water, making it form strong intermolecular forces with water molecules, allowing it to dissolve.

Question 23

Describe how a carboxylic acid can take part in a redox reaction with a metal

Answer 23

RCOOH_(aq) + M_(S) → (RCOO^-)_nMⁿ⁺_(aq) + H₂

Question 24

Describe the reaction of a carboxylic acid with a metal oxide

Answer 24

2RCOOH_(aq) + MO_(s) → (RCOO^-)₂M²⁺_(aq) + H₂O_(l)

Question 25

Describe the reaction of a carboxylic acid with an alkali

Answer 25

NRCOOH_(aq) + M(OH)_n → (RCOOH^-)_nMⁿ⁺_(aq)+ H₂O_(l)

Question 26

Describe the reaction of a carboxylic acid with a carbonate

Answer 26

2RCOOH_(aq) + MCO₃ → (RCOO^-)₂M²⁺_(aq) + H₂O_(l) + CO_2(g)

Question 27

Describe the test for a carboxyl functional group

Answer 27

As carboxylic acids are the only organic compound acidic enough to react with a carbonate, if a reaction happens with a carbonate and an unknown organic compound, the unknown organic compound must contain a carboxyl functional group.

Question 28

Describe the esterification of carboxylic acids with alcohols

Answer 28

Acid + Alcohol ⇔ Ester + Water Acid catalyst of of a concentrated polyprotic acid, e.g. sulfuric acid, is required.

Question 29

Describe the esterification of an acid anhydrides with alcohols

Answer 29

Acid anhydride + Alcohol/Phenol → Ester + Acid

Question 30

Describe the hydrolysis of esters in hot aqueous acid

Answer 30

Ester + (water) ⇔ Acid + Alcohol The water is from the aqueous acid, e.g. hot aqueous dilute sulfuric acid

Question 31

Describe the hydrolysis of esters in hot aqueous alkali (saponification)

Answer 31

Ester + Alkali ⇔ Salt + Alcohol The salt formed is dependent on the alkali and ester, so if methyl ethanoate and sodium hydroxide is used, then Sodium ethanoate is formed as the salt

Question 32

Describe the formation of acyl chlorides from carboxylic acids

Answer 32

Acid + Thionyl Chloride (SOCl₂) → acyl chloride + SO_2(g) + HCl_(g) This reaction should be carried out in a fume cupboard, as the evolved gasses are harmful

Question 33

Describe the esterfication of acyl chlorides with with alcohols

Answer 33

Acyl chloride + Alcohol → Ester + HCl This reaction should be carried out in a fume cupboard, as HCl is a harmful gas

Question 34

Describe the esterification when using a phenol

Answer 34

As carboxylic acids are not reactive enough, either an acyl chloride or acid anhydride must be used, and they react in the same way as with an alcohol, but a phenyl ester is formed

Question 35

Describe the formation of a carboxylic acid from an acyl chloride

Answer 35

Acyl chloride + water → acid + HCl This is a violent reaction so should only happen within a fume cupboard in a controlled way with extra precautions taken e.g. wearing gloves

Question 36

Describe the formation of primary amides from an acyl chloride

Answer 36

Acyl Chloride + 2Ammonia → Primary amide + Ammonium Chloride

Question 37

Describe the formation of a secondary amide from an acyl chloride

Answer 37

Acyl chloride + 2Primary Amine → Secondary Amide + (R)Ammonium Chloride

Question 38

Describe the basicity of amines

Answer 38

Amines behave as bases in their chemical reaction as the lone pari of electrons on the nitrogen can accept a proton, forming a dative covalent bond

Question 39

Describe the reaction of amines with dilute acids

Answer 39

Amines can act as bases so: Amine + Acid → (R)Ammonium Salt

Question 40

Describe the formation of a primary amine and the essential conditions required

Answer 40

Salt formation: Haloalkane + Ammonia → Ammonium Salt Amine formation: Ammonium Salt + Alkaline → Primary Amine + Salt + Water Conditions: Ethanol is used as the solvent for this prevents any substitution of the haloalkane by water to produce alcohols Excess ammonia is used, as this reduces further substitution of the amine group to form secondary and tertiary amines

Question 41

Describe the formation of aromatic amines

Answer 41

The reduction of a Nitrobenzene. It is heated under reflux with tin and concentrated hydrochloric acid (act as reducing agents) to form the ammonium salt, which is then reacted with excess alkali to produce an aromatic amine.

Question 42

What is the general formual of an amino acod?

Answer 42

RCH(NH₂)COOH

Question 43

Describe the reaction of an amino acid with an aqueous alkali

Answer 43

The carboxylic acid group of the amino acid loses a proton, forming a salt and water

Question 44

Describe the esterfication of an amino acid with an alcohol

Answer 44

The carboxylic acid group forms an ester linkage with the alcohol and water molecule in the presence of an acid catalyst

Question 45

Describe how an amide is named

Answer 45

* The number of extra carbons attached to amine part decides how many N's are at the start, e.g. a secondary amide starts with N- * This is followed by the relavent R-groups attached to the nitrogen * The main carbon chain attached after the carbonyl is then said infront of amide, e.g. propanamide

Question 46

Define an optical isomer

Answer 46

An example of sterioisomerism where two molecules are non-superimposable mirror images of each other about a chiral centre

Question 47

Describe how to identify a chiral centre on an organic molecule

Answer 47

The chiral centre will be where four different groups are attached to one carbon atom

Question 48

Describe the formation of polyesters

Answer 48

* Ester linkage formed from a monomer containing both a carboxyl group and a hydroxyl group (giving off water) * Ester linkage formed from a monomer containing both an acyl chloride group and a hydroxyl group (giving off hydrogen chloride gas) * Ester linkage formed from two monomers, with one being a diol and the other being a dicarboxylic acid/diacyl chloride (giving off water or hydrogen chloride gas respectfully)

Question 49

Describe the formation of polyamides

Answer 49

* A monomer of an amino acid forming an amide linkage (giving off water) * A di-acid with a diamine forming an amide linkage (giving off water) * The same as above, but with acyl choride groups instead of carboxyl groups (giving off hydrogen chloride gas instead of water)

Question 50

Describe the base hydrolysis of a polyester

Answer 50

* The carboxylic acid part(s) react with the cation of the base and form a salt * The alcohol part(s) reform the original alcohol used

Question 51

Describe the acid hydrolysis of a polyester

Answer 51

* The carboxylic acid part(s) reform the original carboxyl group * The alcohol part(s) reform the original alcohol * The orginal products are formed as the original polymerisation is a reversable reaction where the acid acts as a catalyst, due to the ester linkage

Question 52

Describe the base hydrolysis of a polyamide

Answer 52

* The carboxylic acid part(s) react with the cation from the base and form a salt * The amine part(s) is reformed by the hydrolysis

Question 53

Describe the acid hydrolysis of a polyamide

Answer 53

* The carboxylic acid part(s) is reformed * The amine part(s) is reformed, but the amine acts as a base, and accepts a proton from the acid

Question 54

Describe how the type of polymerisation can be predicted from looking at the polymer

Answer 54

* Addition polymerisation will have been made from an alkene * Condensation polymerisation will have an ester or amide linkage, and give off a small molecule when being formed

Question 55

Explain the use of C-C formation

Answer 55

Used to increase the length of a carbon chain, allowing longer chained organic compounds to be formed

Question 56

Describe the formation of a nitrile compound from a haloalkane

Answer 56

A nucleophillic substitution reaction with CN^- in ethanol, where the CN^- acts as a nucleophile

Question 57

Show the mechanism for the reaction of a haloalkane compound with a nitrile

Answer 57

Nucleophilic substitution:

Question 58

Describe the reaction of a carbonyl compound and HCN and state why HCN is formed in-situ and how

Answer 58

* A nucleouphilic addition reaction where the CN^- forms an bond where the double was with the carbon the H⁺ from the HCN forms a hydroxyl group with the O * HCN isn't used as its highly toxic * MCN and sulfuric acid, for this gives both the H⁺ and the CN^- required, keeping the rate up, but preventing the toxic HCN from being used

Question 59

Show the mechanism for the reaction of HCN with a carbonyl compound

Answer 59

Nucleophilic addition:

Question 60

Describe the reaction for the reduction of a nitrile

Answer 60

2 mol of H₂ react with one nitrile group in the presence of a nickel catalyst to from a primary amine

Question 61

Describe the reaction for the acid hydrolysis of a nitrile compound

Answer 61

Nitrile forms a carboxylic acid when heated with water and a dilute aqueous acid

Question 62

Describe how to filter under reduced pressure

Answer 62

* Connect the pressure tubing to the vacuum outlet/filter pump and the other end to the Buchner flask * Fit the buchner funnel to the flask ensuring the seal is air tight * Which on the pump/tap * Check for good suction by placing your nad across the top of the funnel * Place the filter paper inside the funnel and wet with the same solvent used in preparing the sold * Slowly pour the reaction mixture from a beaker into the centre of the filter paper * Rince out the beaker with the solvent so that all of the solid crystals collec in the funnel * Rince the crystals with more solvent * Leave crystals under suction to allow them to start to dry

Question 63

Describe the recrystallisation of an organic sold

Answer 63

* Warm the solvent in a water bath/heating mantle (only a bunsen burner if not flammable) * Slowly add the solvent to the impure solid until it dissolves in the solvent, and make sure to add the minimum amount required * Allow the solution to cool and allow crystals to form * If crystals do not appear, you may need to scratch the edge of the beaker with a glass rod * Filter crystals under reduced pressure to obtain the dry solid

Question 64

Describe how to determine the melting point of an organic solid

Answer 64

* Place your sample into a closed end capillary tube * Place the capillary tube into the sample hole in the apparatus, with a thermometer to * Rapidly heat the sample whilst looking through the window * Note the temperature is at which it melts * Repeat the experiment again after allowing equipment to cool, but slowly heat the temperature when approaching the temperature to get a more accurate temperature

Question 65

Explain how to interpret TLC chromatograms

Answer 65

* Calculate R_f values for all compounds analysed * Compare with known R_f values

Question 66

Describe how to carry out a TLC

Answer 66

* On the TLC plate, draw a line with a pencil and draw a cross where you will put a sample * Using a capillary tube, spot a few drops of the sample onto the TLC plate * Place the TLC plate in a 'chromatography tank', which is a beaker filled with a small depth of solvent, and covered on top, e.g. with cling film. * Just before the solvent reaches the top, remove the plate and draw a line where the solvent front reached * Place in a beaker with iodine crystals to allow all spots to form, which R_f values can be calculated

Question 67

Describe how the retention time in gas chromatography can be interpreted

Answer 67

* The shorter the retention time, the less interaction it had with the stationary phase compared to the mobile phase * Each compound has a slightly different retention time, so can be used to determine which compound is present

Question 68

Describe how the amounts and proportions of compounds in a mixture can be determined in gas chromatography

Answer 68

* Obtain gas chromatograms for different standard solutions * Plot a calibration curve for the concentration against area under the peak * Obtain gas chromatograms for the sample given under the same conditions * Compare peak integration values with the calibration curve to work out concentration of gases in the sample

Question 69

What does the number of peaks in carbon-13 NMR tell you about the compound?

Answer 69

The number of carbon environments present in the compound

Question 70

What does the number of groups of peaks in proton NMR tell you about the compound?

Question 71

Explain the use of the peak integration values in proton NMR

Answer 71

The relative number of protons present in that environment

Question 72

Explain the use of splitting patterns in proton NMR

Answer 72

The number of peaks will be one more than the number of protons attached to adjacent atoms

Question 73

Describe the use of tetramethylsilane in NMR

Answer 73

The TMS is used as the standard for chemical shift measurements, so is what all chemical shifts are compared to.

Question 74

Explain why deuterated solvents are used when running an NMR spectrum

Answer 74

For the proton would come up on the spectrum, but the deutranium molecule which replaces it and this does not come up on the spectrum, so will not interfere with results

Question 75

Describe and explain the effect on the NMR spectrum of a compound when mixed with D₂O

Answer 75

* The deutranium molecule can exchange with a proton which is part of O-H or N-H * This means that O-H and N-H carbon environment will not show up, allowing them to be identified